In light of the evermore stringent emissions regulations that are planned to take effect over the next few years, including California Low Emission Vehicle II (LEV II), Federal USA EPA Tier 2 and European Union EU-IV, pre-catalyst engine-out HC emissions, especially during cold start and warm-up, are attracting significant efforts in research and development. This is due in large part to the fact that as much as 80 percent of the total hydrocarbon emissions produced by a typical, modern light-duty vehicle during the Federal Test Procedure (FTP) can occur during the first 120 seconds of the test.
These high levels of emissions are largely attributable to cold engine and exhaust component temperatures. Specifically, cold engine components necessitate fuel-rich operation, in which the excess fuel is used to compensate for the portion of fuel that has attached to the walls of the intake system and combustion chamber and, thus, is not readily combusted. In addition, a cold three-way catalyst cannot reduce a significant amount of the unburned hydrocarbons that pass through the engine during cold-start. As a result, high concentrations of unburned hydrocarbons are emitted from the tailpipe. It is understood that the over-fueling associated with excessive hydrocarbon emissions during cold-start could be eliminated through the use of gasoline vapor rather than liquid gasoline.
A variety of systems have been devised to supply fine liquid fuel droplets and air to internal combustion engines that work relatively well after engine warm-up. These systems either supply fuel directly into the combustion chamber (direct injection) or utilize a carburetor or fuel injector(s) to supply the mixture through an intake manifold into a combustion chamber (indirect injection). In currently employed systems, the fuel-air mixture is produced by atomizing a liquid fuel and supplying it as fine droplets into an air stream.
In conventional spark-ignited engines employing port-fuel injection, the injected fuel is vaporized by directing the liquid fuel droplets at hot components in the intake port or manifold. Under normal operating conditions, the liquid fuel films on the surfaces of the hot components and is subsequently vaporized. The mixture of vaporized fuel and intake air is then drawn into the cylinder by the pressure differential created as the intake valve opens and the piston moves towards bottom dead center. To ensure a degree of control that is compatible with modern engines, this vaporizing technique is typically optimized to occur in less than one engine cycle.
Under most engine operating conditions, the temperature of the intake components is sufficient to rapidly vaporize the impinging liquid fuel droplets. However, as indicated, under conditions such as cold-start and warm-up, the fuel is not vaporized through impingement on the relatively cold engine components. Instead, engine operation under these conditions is ensured by supplying excess fuel such that a sufficient fraction evaporates through heat and mass transfer as it travels through the air prior to impinging on a cold intake component. Evaporation rate through this mechanism is a function of fuel properties, temperature, pressure, relative droplet and air velocities and droplet diameter. Of course, this approach breaks down in extreme ambient cold-starts, in which the fuel volatility is insufficient to produce vapor in ignitable concentrations with air.
In order for combustion to be chemically complete, the fuel-air mixture must be vaporized to a stoichiometric or fuel-lean gas-phase mixture. A stoichiometric combustible mixture contains the exact quantities of air (oxygen) and fuel required for complete combustion. For gasoline, this air-fuel ratio is about 14.7:1 by weight. A fuel-air mixture that is not completely vaporized, or is rich or overly lean, results in incomplete combustion and reduced thermal efficiency. The products of an ideal combustion process are water (H2O) and carbon dioxide (CO2). If combustion is incomplete, some carbon is not fully oxidized, yielding carbon monoxide (CO) and unburned hydrocarbons (HC).
The mandate to reduce air pollution has resulted in attempts to compensate for combustion inefficiencies with a multiplicity of fuel system and engine modifications. As evidenced by the prior art relating to fuel preparation and delivery systems, much effort has been directed to reducing liquid fuel droplet size, increasing system turbulence and providing sufficient heat to vaporize fuels to permit more complete combustion.
Given the relatively large proportion of unburned hydrocarbons emitted during startup, this aspect of light duty vehicle engine operation has been the focus of significant technology development efforts. Furthermore, as increasingly stringent emissions standards are enacted into legislation and consumers remain sensitive to pricing and performance, these development efforts will continue to be paramount. One general class of solutions employed to reduce engine startup emissions involves fuel vaporization. Key practical challenges to providing vaporized fuel include the fact that metering fuel vapor is problematic, and thus most approaches to reducing cold-start emissions focus on metering the fuel as a liquid and then vaporizing it. Heated fuel injector concepts with fuel heaters or vaporizers added on at the outlet of the injector generally suffer from poor atomization and fuel targeting once the heater is turned off. In addition, heated injector and heated impingement plates suffer from an intrinsic design challenge between minimizing the power required to the heating element and minimizing the vaporizer warm-up time. For practical purposes the heating time associated with both heated injectors and heated impingement plates are too long unless excessive electrical power is supplied.
One particular solution to the aforementioned difficulties associated with fuel vaporization implementations involves the use of capillary channels to vaporize fuel. The use of capillary channels offers a number of distinct advantages over other techniques that are directed at supplying vaporized fuel to internal combustion engines. In particular, the high surface area to volume ratio of the capillary channel combined with the relatively low thermal mass of the capillary structure results in fast warm up times (on the order of less than 0.5 seconds) and minimal power requirements (on the order of 150 watts per cylinder) necessary to achieve the desired heating profile. Yet another advantage of capillary channels in connection with fuel vaporization is the fact that the capillary design can be integrated with the functionality of a conventional fuel injector such that a single injector can supply both liquid and vaporized fuel, depending upon the selected emission control strategy.
One form of a capillary channel-based fuel vaporizer is disclosed in U.S. patent application Ser. No. 10/284,180, filed on Oct. 31, 2002, such patent application being the patent application upon which this patent application is based. In that application, a fuel system for use in an internal combustion engine is disclosed and a preferred form includes a plurality of fuel injectors, each injector including (i) at least one capillary flow passage, the at least one capillary flow passage having an inlet end and an outlet end, (ii) a heat source arranged along the at least one capillary flow passage, the heat source operable to heat a liquid fuel in the at least one capillary flow passage to a level sufficient to convert at least a portion thereof from the liquid state to a vapor state, and (iii) a valve for metering fuel to the internal combustion engine, the valve located proximate to the outlet end of the at least one capillary flow passage, a liquid fuel supply system in fluid communication with the plurality of fuel injectors, a controller to control the power supplied to the heat source of each of the plurality of fuel injectors to achieve a predetermined target temperature, the predetermined target temperature operable to convert a portion of liquid fuel to the vapor state; means for determining engine air flow of the internal combustion engine, and a sensor for measuring a value indicative of degree of engine warm-up of the internal combustion engine, the sensor operatively connected to the controller; and wherein the portion of liquid fuel to be converted to the vapor state is controlled with reference to sensed internal combustion engine conditions to achieve minimal exhaust emissions.
The fuel system disclosed in the patent application upon which this patent application is based is effective in reducing cold-start and warm-up emissions of an internal combustion engine. Efficient combustion is promoted by forming an aerosol of fine droplet size when the substantially vaporized fuel condenses in air. The vaporized fuel can be supplied to a combustion chamber of an internal combustion engine during cold-start and warm-up of the engine and reduced emissions can be achieved.
The patent application upon which this patent is based also discloses a method for controlling the fuel system and delivering fuel to an internal combustion engine for a fuel system including at least one fuel injector having at least one capillary flow passage, a heat source arranged along the at least one capillary flow passage, the heat source capable of heating a liquid fuel in the at least one capillary flow passage to a level sufficient to convert at least a portion thereof from the liquid state to a vapor state, and a valve for metering fuel to the internal combustion engine, the valve located proximate to an outlet end of the at least one capillary flow passage. The method includes the steps of determining engine air flow of the internal combustion engine, measuring a value indicative of degree of engine warm-up of the internal combustion engine, determining a portion of liquid fuel to be converted to the vapor state by the at least one capillary flow passage, the determining step employing the measured values, controlling power supplied to the heat source of the at least one fuel injector to achieve a predetermined target temperature, the predetermined target temperature operable to convert the portion of liquid fuel to the vapor state so determined and delivering the fuel to a combustion chamber of the internal combustion engine and wherein the portion of liquid fuel to be converted to the vapor state is determined to achieve minimal exhaust emissions.
According to one preferred form described in that patent application, the capillary flow passage can include a capillary tube and the heat source can include a resistance heating element or a section of the tube heated by passing electrical current therethrough. The fuel supply can be arranged to deliver pressurized or non-pressurized liquid fuel to the flow passage. The apparatus can provide a stream of vaporized fuel that mixes with air and forms an aerosol that can be carried by an air stream, regardless of the flow path, into the combustion chamber.
As further described in the patent application upon which this patent application is based, the target temperature of the capillary is determined through the use of a control algorithm designed to achieve an appropriate target setpoint. The target setpoint is the ratio of the hot resistance of the capillary to the cold (unheated) resistance of the capillary (R/Ro). The ratio R/Ro, in turn, corresponds to a desired bulk capillary temperature.
Unfortunately, over time, the material properties of the capillaries may change as a result of various physical phenomena, including annealing of the capillary material and deposits buildup on the interior capillary walls. As a result of these changes, the value of Ro (the cold, unheated resistance of the capillary) may change accordingly. As a result, the bulk capillary temperature that corresponds to a given value of R/Ro may also change over time. Left unaddressed, sub-optimal operating characteristics may result. These include situations in which (i) insufficient heat is supplied to the capillary resulting in poor fuel vaporization; (ii) insufficient heat is supplied to the capillary resulting in no fuel vaporization; (iii) the capillary is overheated resulting in an increased power requirement with respect to the capillary; and (iv) the capillary is overheated which may result in mechanical failure of the capillary including melting, cracking or breaking.
These failures are undesirable for a number of obvious reasons. In addition, the California Air Resources Board (CARB) mandates that for 2007, emissions related components are required to operate properly for at least 150,000 miles. This is well beyond the range at which such a failure resulting from natural capillary material changes may occur unless capillary heat is carefully controlled in response to the physical material changes in the capillaries.
Further, as per CARB and EPA mandates, any faults in the operation of the fuel injector must be detectable by the On-Board Diagnostics-II (OBD-II) system of the vehicle. Unfortunately, it has been in the past for various reasons, difficult to comply with vehicle OBD-II requirements in a way that is inexpensive to implement and in which the solution remains robust throughout the life of the component.
Proper operation of the capillary channel based fuel injector discussed above, in a situation when no heat is supplied to the capillary (i.e. operation as a conventional fuel injector) during fully warmed engine conditions, is inherently detectable through modern vehicle architectures that employ electronic fuel injection, electronic engine control (EEC), and exhaust gas oxygen sensors. The duty cycle of the electronic fuel injector, as requested by the EEC, provides an indication of the target amount of fuel that should be supplied to the engine. The exhaust gas oxygen sensor(s) provide an indication of the fuel that actually has been supplied by the injectors to the engine. Thus a straightforward OBD-II check of the conventional fuel injector operation may be based upon a comparison of the fuel injector duty cycle and the signal from the exhaust gas oxygen sensor.
While operational status is readily detectible in the non-heated mode of the capillary channel-based injector, operational fault detection is not as straightforward in the case where the capillary is being heated. Since the volumetric flow rate of the fuel varies with temperature the above discussed fault detection scheme does not work during operational modes in which the capillary is being heated to some degree.